The disclosed invention generally relates to a system for extending the spectral response of spectral analyzing and direction indicating apparatus which utilize diffracting elements.
Diffraction elements (e.g., diffraction gratings) may be utilized to provide information as to the incident direction and wavelength of incident essentially collimated radiation. Such use of diffraction elements is based on the diffraction grating equation: EQU Sin .theta.=nL/d-sin I (Equation 1)
where .theta. is the angle of diffraction, I is the angle of incidence, n is the order of diffraction, L is the wavelength, and d is the distance between adjacent rulings on the diffraction grating. The angles .theta. and I are measured relative to the normal to the plane of the diffraction element.
Examples of systems which utilize diffraction elements to determine simultaneously both incident direction and wavelength of incident essentially collimated radiation are set forth in U.S. Pat. No. 4,624,566, issued on Nov. 25, 1986, by G. A. Shifrin et al.; U.S. Pat. No. 4,674,871 issued on June 23, 1987, by G. A. Shifrin; U.S. Pat. No. 4,682,888 issued on July 28, 1987, by J. M. Welner; and U.S. Ser. No. 737,114, filed on May 23, 1985, by G. A. Shifrin.
For ease of reference, spectral analyzing and direction indicating systems which utilize diffraction elements shall be referred to herein as diffraction based detection systems.
The use of diffraction elements provides for possible ambiguities. For example, consider two rays R1 and R2 respectively having wavelengths of L1 and L2, where L2 is two times L1, and which have the same incidence angle. On the basis of wavelength alone, the second order diffraction of the ray R1 would be indistinguishable from the first order diffraction of the ray R2. This circumstance would occur, however, only if the detector array (suitably arranged to intercept the diffracted rays) was designed with sufficient field of view to intercept both R1 and R2.
In order to avoid the foregoing ambiguities, the spectral interval over which the diffraction elements can operate is generally limited to one octave. Such limitation may be achieved by limiting the relative wavelength interval falling on a diffraction grating to no more than a factor of two (which corresponds to one octave), or by restricting the angular range intercepted by a detector array to that occupied only by diffracted rays of the first order.
The foregoing limitations on spectral interval can be costly for systems which must provide continuous coverage over a given spectral interval greater than one octave. Specifically, a group of subsystems would be required where each subsystem would cover an interval of no more than one octave.